This invention relates to electrical circuits and more specifically to a system and method for generating reference currents, such as used in biasing signal amplifiers within an integrated circuit.
Integrated circuits, whether digital or analog in form, require reference currents. A reference current is a current source generated by the integrated circuit for the purpose of operating devices of the integrated circuit in a manner that minimizes the effects of variation in power supply, temperature, and fabrication process at a particular location within the integrated circuit. For example, a high speed differential amplifier used in an off-chip driver of a communication circuit needs a reference current to drive signals with required fixed amplitude onto a signal line towards a remote receiver, despite variations which occur in power supply, temperature, resistance values and fabrication process relative to particular locations of the chip.
As shown in FIG. 1, an exemplary high speed differential amplifier 10 drives differential outputs OUTP and OUTN based on the voltages of input signals INN and INP presented thereto. The differential amplifier 10 includes a “tail” transistor 20 which is coupled in mirror configuration to a first transistor 22 such that the tail transistor 20 generates a tail current It which is proportional to the reference current Ir through the first transistor 22. The tail current It is used to pull down one of the outputs OUTP or OUTN as a voltage drop across one of the on-chip load resistors RL by the quantity ItRL, based on the inputs INN and INP presented to the differential amplifier. When an output OUTP or OUTN is pulled down in use, the voltage drop across the corresponding one of the on-chip load resistors RL is required to be of fixed amplitude. Since the values of the on-chip load resistors RL vary with temperature and the fabrication process conditions, it will be understood that the reference current Ir, from which the tail current is mirrored, must not be constant, but rather must vary in a way to compensate for such temperature and process-related variations in resistance.
On the other hand, some circuits, which do not use on-chip resistors as load elements, are also required to provide output signals of fixed amplitude. For example, many different configurations of differential amplifiers are available which include transistors rather than resistors as load elements. In such cases, a reference current is needed which does not vary according to changes in an on-chip resistance, but rather, is independent from the variability of on-chip resistances.
Other problems of existing reference current generators are the chip area and power consumed by the placement of multiple independent reference current generators at different locations on a chip, such reference current generators including many elements that are duplicative. In addition, variations in the fabrication processing at such different chip locations may result in local variations in the generated reference currents. Therefore, a reference current generator system is desired which reduces demands on chip area and power consumption by eliminating duplicative elements and which provides uniform reference currents.
It would further be desirable for a reference current generator system to centrally generate a plurality of reference currents, and then distribute the reference currents to a plurality of different locations on a chip where a set of local reference currents are regenerated from the distributed reference currents and then used.